In recent years, information terminals such as smartphones are being used by a growing number of users. These information terminals are configured to be capable of making phone calls or connecting to the Internet via wireless channels provided by 3G (3rd Generation), Wi-Fi (Wireless Fidelity) or other wireless communication technologies. On the other hand, buildings such as houses are usually equipped with wired telephone lines. However, users of the information terminals tend to make calls over 3G wireless channels or the like even in buildings equipped with telephone lines, and thus, the communication/call costs tend to be high.
To address such a problem, it is proposed to provide a mobile phone having a battery box mount provided with an external interface for connection with a control unit, such that an adapter to a cordless telephone system can be detachably attached to the battery box mount, where the adapter includes a wireless unit, an antenna, a battery unit and an adapter-side interface for connection with the external interface and performs transmission and reception to and from the base unit of the cordless telephone system by use of radio waves (JPH04-334133A).
Further, there is disclosed a mobile phone including a first transmitting and receiving unit that uses a first frequency used by a cordless telephone system, a second transmitting and receiving unit that uses a second frequency allocated to general cellular phones, a switching means for switching the first and second frequencies when placing a call, and a monitoring means that simultaneously monitors the first and second frequencies of the radio waves received by the first and second transmitting and receiving units (JPH05-244088A).
To achieve a smartphone having a function of connecting to the telephone line by use of a cordless telephone system, it may be conceived to provide a battery-operated handset relaying communication between the smartphone and the base unit of the cordless telephone system, where the user of the smartphone carries the handset together with the smartphone. In such a case, it is preferred to make the handset as small in size as possible in view of portability. However, reduction in size of the handset tends to limit the battery capacity of the handset, and therefore, it is required to reduce the power consumption of the handset.
It may be also conceived to adapt the handset to receive electric power from the handset from the battery of the smartphone. However, it is already an important issue that smartphones tend to consume battery power quickly. Namely, smartphones have a computation power comparable to that of PCs (personal computers) and a relatively large screen size, and these result in a large power consumption. Thus, if the handset is configured to receive electric power from the battery of the smartphone, the power of the battery of the smartphone will be consumed even more quickly. Therefore, in such a case also, it is required to reduce the power consumption of the handset.
JPH04-334133A discloses an adapter to a cordless telephone system detachably mounted to a mobile phone and relaying communication between the mobile phone and the base unit of the cordless telephone system. However, there is no disclosure in this document of a detailed mode of operation of the adapter, which is a handset of the cordless telephone system, and it is difficult to achieve reduction in the power consumption with the structure disclosed in this document.
The technology disclosed in JPH05-244088A relates to “simultaneously monitoring the first and second frequencies of the radio waves received by the first and second transmitting and receiving units,” and thus, in the mobile phone based on this technology, multiple transmitting and receiving units operate simultaneously, and this may rather increase the power consumption.